Association between Mn-reducing root bacteria and NaCl applications in suppression of Fusarium crown and root rot of asparagus

Ten field tests were conducted to determine if the suppression of Fusarium crown and root rot of asparagus by NaCl was associated with changes in the densities and characteristics of root-colonizing bacteria. A total of 1,114 strains of root-colonizing bacteria were isolated from NaCl-treated and nontreated feeder roots (80 to 200 per test) and rated for their ability to reduce Mn on a Mn-dioxide medium, for UV fluorescence, and for antagonism against Fusarium oxysporum and F. proliferatum. In addition, bacterial densities on roots, levels of Mn in roots, and marketable yield were measured. In seven out of 10 tests, NaCl-treated roots had a greater percentage of Mn-reducing bacteria than control roots. Overall, the combined data revealed a significant increase in the small fraction of Mn-reducing bacteria on NaCl-treated roots compared to control roots (P = 0.006). In five out of six tests, roots from NaCl-treated plots had an average of 19% more Mn per gram of root than roots from control plots (P = 0.08). The NaCl treatment did not affect the densities of bacteria on roots or the percentage of strains that were antagonistic to the Fusarium spp. The percentage of strains that fluoresced under UV light on King's B medium was significantly higher in half of the tests and, overall, was significant at P = 0.097. Fluorescent pseudomonads were more likely to be Mn-reducing strains than were nonfluorescing bacteria (P = 0.019). Twenty-eight out of 30 Mn-reducing strains were Pseudomonas spp. (64% fluorescent pseudomonads) and two were Serratia spp. Only seven out of 20 Mn-reducing strains suppressed disease on asparagus transplants in greenhouse tests. The disease-suppressive strains of Serratia spp. and P. corrugata were associated with higher levels of Mn in the roots of asparagus transplants grown in noninfested soils than in nontreated roots, but there were other Mn-reducing strains that did not affect disease or Mn levels. The interrelationships among NaCl treatments, suppression of Fusarium crown and root rot, and increases in Mn-reducing rhizobacteria tended to support the hypothesis that increased Mn availability contributes to disease resistance in asparagus.